This invention relates to a method of preparing a bonded composite and to a bonded composite formed thereby. In particular, the method of this invention includes forming an uncured composite containing a prepolymer composition and curing the prepolymer composition. Further, this invention relates to a prepolymer composition. Bonded composites are prepared by bonding two or more substrates together with an interposed bonding composition. The bonding composition must adhere well to the substrates and be resistant to forces that lead to separation.
Some bonded composites are made of relatively thin, flat layers; such bonded composites are generally known as laminates. The method of this invention is useful for preparing various types of bonded composites, including laminates, especially flexible laminates. Laminates are used to provide packaging which is light-weight and flexible. Typically, laminates are formed from combinations of various polymeric films including polymeric substrates with low surface energies and metal foils bonded together by a bonding composition. It is desirable to use the bonding composition at a low application weight to minimize the weight of the laminate, to maintain flexibility, and to minimize cost.
New bonding methods are desired which allow the preparation of bonded composites from opaque substrates. Multilayered laminates are also desired which may be formed with a single cure step. One approach to these goals has been to bond the substrates with a bonding composition that is applied as a liquid and then cured by exposure to ultraviolet (uv) radiation or electron beam (e-beam) radiation. However, cure with uv radiation often requires the use of photoinitiators, and photoinitiators or the fragments of photoinitiators resulting from photolysis are generally low molecular weight materials which may adversely affect the properties of the laminate. Compositions that are curable by exposure to electron beam radiation generally do not require photoinitiators.
One approach to making bonding compositions curable with electron beams is in U.S. patent application Ser. No. 10/135,258, which discloses the use of bonding compositions formed from prepolymers that contain tackifying resins, polymerizable oligomers, and monomers; these bonding compositions are then cured by electron beam radiation. This approach requires special, potentially expensive, materials. Another approach is in JP 59105068, which uses formulations containing conventional polyester resins for electron beam curable adhesives.
However, it is desirable to further improve bonded composites by reducing the cost and performance of the bonding composition. We have found, surprisingly, that bonded composites can be made by using electron beam radiation to cure relatively inexpensive bonding compositions that contain uncurable polyester resins with acid value of 2 mg KOH/g of resin or greater.